In exclusively electronically controlled powershift transmissions, after a load take up by the engaged gearshift element, it must be checked whether the disconnected gear-shift element has also reliably disengaged. This can be done by monitoring the slip that occurs on the old gearshift element. The slip is calculated according to a turbine speed and an output speed. If, after a certain period of time, no slip results after the switching torque has been impressed on the shift element to be engaged, the old gear is retained in order to prevent a blockage of the transmission (EP-A 0 436 977).
For adjusting the hydraulic pressure in a shift element of an automatic transmission, DE-A 32 05 767 has disclosed a method in which the input speed and the output speed of the transmission are determined, and the pressure is adjusted so that the reduction ratio of the transmission during a change of gear follows a preset time function. The whole shifting operation preferably takes place in two steps wherein the pressure is controlled up to a free-wheel point, that is, the point at which the friction elements coordinated with the old gear disengage, and thereafter follows the regulation to a preset reduction ratio or a change of the reduction ratio. If the free-wheel point in the transmission is not reached, a time control ensures that the pressure is raised in a ramp-like manner until reaching the free-wheel point.
The effects upon the output torque, which result when the old shift element does not disengage, in particular are the following:
During a high engine torque, there is the danger of the torque on the transmission output falling to a very low value. This effect is especially negative, for example, in the case of an overtaking operation where the full engine power is needed or expected. During a slight engine torque, there results, instead of a positive traction torque, a substantially equally strong thrust torque. Especially on a smooth road, this could lead to the blocking of the wheels at least when the full pressure would be given to the shift element to be engaged in case the time step is exceeded.